Pretreatment method for increasing conversion of reforming catalyst

ABSTRACT

A pretreatment process is disclosed for increasing conversion and reducing the fouling rate of reforming catalysts wherein the catalyst is pretreated at a temperature from 1025° F. to 1275° F. in a reducing atmosphere prior to contacting the catalyst with a hydrocarbon feed in the presence of hydrogen.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. application Ser. No.976,786, filed Nov. 16, 1992 and now abandoned.

BACKGROUND OF THE INVENTION

The present invention concerns a pretreatment method useful forincreasing the conversion and lowering the fouling rate of a reformingcatalyst.

Catalytic reforming is a well-known process that is used for raising theoctane rating of a naphtha for gasoline. The reactions that occur duringreforming include: dehydrogenation of cyclohexanes, dehydroisomerizationof alkylcyclopentanes, dehydrocyclization of acyclic hydrocarbons,dealkylation of alkylbenzenes, isomerization of paraffins, andhydrocracking of paraffins. The hydrocracking reaction should besuppressed because that reaction lowers the yield of hydrogen and lowersthe yield of liquid products.

Reforming catalysts must be selective for dehydrocyclization, in orderto produce high yields of liquid product and low yields of light gases.These catalysts should possess good activity, so that low temperaturescan be used in the reformer. Also, they should possess good stability,so that they can maintain a high activity and a high selectivity fordehydrocyclization over a long period of time.

While most reforming catalysts contain platinum on an alumina support,large-pore zeolites have been proposed as supports. These large-porezeolites have pores large enough for hydrocarbons in the gasolineboiling range to pass through. Commercial application of zeoliticreforming catalysts have thus far been very limited, although certaincatalysts comprising a large-pore zeolite containing at least one GroupVIII metal have a very high selectivity for dehydrocyclization.

It is known that reforming catalysts require pretreatment prior toutilizing these catalysts for reforming naphtha feedstocks. For example,U.S. Pat. No. 4,517,306 issued to Waldeen Buss on May 14, 1985 claims acomposition comprising: (a) a type L zeolite; (b) at least one GroupVIII metal; and (c) an alkaline earth metal selected from the groupconsisting of barium, strontium and calcium, wherein said composition isreduced in a hydrogen atmosphere at a temperature of from 480° C. to620° C. (896° to 1148° F.). It is preferred that the composition bereduced at a temperature from 550° to 620° C. (1022° to 1148° F.).

U.S. Pat. No. 4,539,304 issued on Sep. 3, 1985 to Field discloses atwo-step pretreatment process for increasing the conversion of reformingcatalysts wherein the catalyst is first treated at a temperature of from120° C. (248° F.) to 260° C. (500° F.) in a reducing gas. In the secondstep, the temperature of the catalyst is maintained at 370° C. (698° F.)to 600° C. (1112° F.) in a reducing atmosphere.

U.S. Pat. No. 4,539,305 issued on Sep. 3, 1985 to Wilson et. al.discloses a pretreatment process for enhancing the selectivity andincreasing the stability of a reforming catalyst comprising a large-porezeolite containing at least one Group VIII metal. The catalyst isreduced in a reducing atmosphere at a temperature of from 250° C. (482°)to 650° (1202° F.). The reduced catalyst is subsequently exposed to anoxygen-containing gas and then treated in a reducing atmosphere at atemperature of from 120° C. (248° F.) to 260° C. (500° F.). Finally, thecatalyst is maintained at a temperature of from 370° C. (698° F.) to600° C. (1112° F.) in a reducing atmosphere. Preferably, the firstreduction step is carried out in the presence of hydrogen.

U.S. Pat. No. 5,155,075 issued to Innes et. al. shows an initialcatalyst reduction at 300° F. to 700° F., followed by a temperature rampup to a final hydrogen treatment temperature between 900° F. and 1000°F.

U.S. Pat. No. 5,066,632 issued on Nov. 19, 1991 to Baird et. al.discloses a process for pretreating a catalyst useful for reforming anaphtha wherein the catalyst is calcined at temperatures in excess of500° F., preferably at temperatures ranging from 500° F. to about 750°F. in air or in atmospheres containing low partial pressures of oxygenor in a non-reactive or inert gas such as nitrogen. The catalyst is thencontacted with a dry hydrogen-containing gas at a temperature rangingfrom about 600° F. to about 1000° F., preferably from about 750° F. toabout 950° F., at a hydrogen partial pressure ranging from about 1atmosphere to about 40 atmospheres, preferably from 5 atmospheres toabout 30 atmospheres.

European Patent Application Publication Number 243,129 discloses acatalyst activation treatment with hydrogen at temperatures from 400° C.(752° F.) to 800° C. (1472° F.), preferably from 400° C. (752° F.) to700° C. (1292° F.) for a catalyst used for cracking a hydrocarbonfeedstock. The treatment pressure may vary from 100 to 5,000 MPa but ispreferably from 100 to 2,000 MPa. A carrier gas which contains 1-100%v/v, preferably from 30-100% v/v, of hydrogen is used.

U.S. Pat. No. 4,717,700 issued to Venkatram et. al. discloses a methodfor drying a zeolite catalyst by heating while in contact with a gas.The rate of catalyst temperature increase is controlled so as to limitthe rate of water evolution from the catalyst and the water vaporconcentration in the gas. The gas used to heat the catalyst is graduallyincreased in temperature at about 28° C. per hour. The moisture level ofthe effluent gas is preferably between 500 and 1500 ppm during thedrying step. The catalyst drying method with a subsequent reduction withhydrogen wherein the temperature is raised to a maximum temperature of450° C. is exemplified in Example 1.

Austrian Patent Specification No. 268,210 relates to a metal-chargedzeolite molecular sieve, which is suitable as a catalyst for theconversion of hydrocarbons. Methods for preparing the catalyst aredescribed. It is disclosed that the catalyst prepared by such methodsusually has a high water content and that it is desirable to activatethe catalyst before use since the catalyst is sensitive to water. Therecommended activation process comprises: 1) slow heating of thecatalyst in air at 300° to 600° C., preferably 500° C.; followed by 2)slow heating of the catalyst from room temperature to approximately 500°C. in a current of hydrogen gas under atmospheric pressure.

A pretreatment process of Pt-Al₂ O₃ catalysts in hydrogen in thetemperature range of 450° C. (842° F.) to 600° C. (1112° F.) isdisclosed in Journal of Catalysis (1979); Vol. 59, p. 138 (P. G. Menonand G. F. Froment). The effect of catalyst reduction temperature on theconversion of n-pentane and n-hexane using Pt-Al₂ O₃ catalysts isdisclosed. For the Pt-Al₂ O₃ catalyst reduced at 400° C. (752° F.),hydrogenolysis is the main reaction; whereas for the Pt-Al₂ O₃ catalystreduced at 600° C. (1112°), the hydrogenolysis and total activity areconsiderably suppressed. This reference specifically discloses theeffect of a hydrogen pretreatment process on Pt-Al₂ O₃ catalysts anddoes not disclose the effect of hydrogen pretreatment on zeoliticcatalyst.

Additionally, the effects of hydrogen pretreatment of the Pt-Al₂ O₃catalyst with respect to isomerization is disclosed. The activity fordehydrocyclization was not increased.

Prior art processes have observed both a reduced catalytic activity andreduced hydrogen chemisorption for catalysts which have been reduced attemperatures in excess of 500° C. Furthermore, there has been no clearunderstanding of the phenomena which occur during high temperaturecatalyst reduction. Thus, reduction at high temperatures may result instrongly chemisorbed hydrogen, may cause loss of spillover hydrogenaltering the local charge transfer from the support to the metal at theparticle boundary, may induce changes in morphology of the metalcrystallite, or may affect reduction of the support resulting in theformation of an alloy with atoms from the support.

SUMMARY OF THE INVENTION

The present invention is a process for increasing the conversion andlowering the fouling rate of large-pore zeolitic reforming catalystsusing a pretreatment process. The catalyst is treated in a reducing gasat a temperature of from 1025° F. to 1275° F.

Preferably, the pretreatment process in the range of 1025° F. to 1275°F. occurs in the presence of hydrogen at a pressure of from 0 to 300psig for from 1 hour to 120 hours. Generally, the higher the treatmenttemperature employed, the shorter the treatment time needed to achievethe desired effect.

More preferably, the catalyst is reduced with dry hydrogen viatemperature-programmed steps, with the treatment of the presentinvention occurring at the final temperature of from 1025° F. to 1275°F. The procedure of the present invention which occurs in thetemperature range of from 1025° F. to 1275° F. is considered andreferred to as a "treatment" of the catalyst as opposed to a"reduction", because the catalyst has already generally been reduced atthe lower temperatures prior to reaching the treatment temperature ofthe present invention.

Among other factors, we have found that large-pore zeolitic catalystswhich have been pretreated in a reducing gas in the high temperaturerange of from about 1025° F. to 1275° F. is found to have a lowerfouling rate and improved activity, and have a longer run life. Inparticular, this catalyst exhibits a longer run life with heavierfeedstocks than with similar catalysts using other pretreatmentprocesses. For example, if a L zeolite catalyst is pretreated byconventional methods, run lengths with feeds containing C₉ +hydrocarbons are generally short. The pretreatment procedure of thisinvention, however, makes it practical to process feedstocks containingas much as 5-15 wt % C₉ + hydrocarbons.

Thus, in spite of the disadvantages that the prior art recognizes withrespect to high temperature catalyst reduction, the present inventorshave discovered an advantageous high temperature catalyst treatmentmethod. In particular, the present invention has surprisingly found thata high temperature treatment (i.e., at 1025° F. to 1275° F.) will resultin a catalyst with a reduced fouling rate and sufficient catalyticactivity to yield a longer run life, particularly if the temperatureincrease during reduction is performed in a gradual ramping or stepwisefashion, and if the water content of the effluent gas is kept as low aspossible during the high temperature treatment range. Even catalyststhat are on balance non-acidic still contain a few residual acidicsites. This high temperature treatment regimen is believed to reduce thenumber of acid sites on the catalyst, and thereby reduce side reactionswhich lead to the formation of coke. The improved fouling rate andconversion activity of the catalyst also allow for more beneficial usewith a heavier feedstock.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 of the Drawing is a graphical representation of hydrogen uptakeonto catalyst as a function of temperature.

FIG. 2 of the Drawing is a graphical representation of the fouling ratesobserved for different temperature treatments.

DETAILED DESCRIPTION OF THE INVENTION

In its broadest aspect, the present invention is a process forincreasing the conversion and/or lowering the fouling rate of large-porezeolitic reforming catalysts using a pretreatment process. This catalystis treated in a reducing gas at a temperature of from 1025° F. to 1275°F.

Preferably, the pretreatment process occurs in the presence of hydrogenat a pressure of from 0 to 300 psig and a temperature of from 1025° F.to 1275° F. for from 1 hour to 120 hours, more preferably for at least 2hours, and most preferably at least 4-48 hours. More preferably, thetemperature is from 1050° F. to 1250° F. In general, the length of timefor the pretreatment will be somewhat dependent upon the final treatmenttemperature, with the higher the final temperature the shorter thetreatment time that is needed.

For a commercial size plant, it is necessary to limit the moisturecontent of the environment during the high temperature treatment inorder to prevent significant catalyst deactivation. In the temperaturerange of from 1025° F. to 1275° F., the presence of moisture is believedto have a severely detrimental effect on the catalyst activity, and ithas therefore been found necessary to limit the moisture content of theenvironment to as little water as possible during said treatment period,to at least less than 200 ppm.

In one embodiment, in order to limit exposure of the catalyst to watervapor at high temperatures, it is preferred that the catalyst be reducedinitially at a temperature between 300° F. and 700° F. After most of thewater generated during catalyst reduction has evolved from the catalyst,the temperature is raised slowly in ramping or stepwise fashion to amaximum temperature between 1025° F. and 1250° F.

The temperature program and gas flow rates should be selected to limitwater vapor levels in the reactor effluent to less than 200 ppm and,preferably, less than 100 ppm when the catalyst bed temperature exceeds1025° F. The rate of temperature increase to the final activationtemperature will typically average between 5° and 50° F. per hour.Generally, the catalyst will be heated at a rate between 10° and 25°F./h. It is preferred that the gas flow through the catalyst bed (GHSV)during this process exceed 500 volumes per volume of catalyst per hour,where the gas volume is measured at standard conditions of oneatmosphere and 60° F. GHSV's in excess of 5000 h⁻¹ will normally exceedthe compressor capacity. GHSV's between 600 and 2000 h⁻¹ are mostpreferred.

The pretreatment process of the present invention occurs prior tocontacting the reforming catalyst with a hydrocarbon feed.

The large-pore zeolitic catalyst is generally treated in a reducingatmosphere in the temperature range of from 1025° F. to 1275° F.Although other reducing gasses can be used, dry hydrogen is preferred asa reducing gas. The hydrogen is generally mixed with an inert gas suchas nitrogen, with the amount of hydrogen in the mixture generallyranging from 1%-99% by volume. More typically, however, the amount ofhydrogen in the mixture ranges from about 10%-50% by volume.

The reducing gas entering the reactor should contain less than 100 ppmwater. It is preferred that it contain less than 10 ppm water. In acommercial operation, the reactor effluent may be passed through a driercontaining a desiccant or sorbent such as 4 Å molecular sieves. Thedried gas containing less than 100 ppm water or, preferably, less than10 ppm water may then be recycled to the reactor.

The feed to the reforming process is typically a naphtha that containsat least some acyclic hydrocarbons or alkylcyclopentanes. This feedshould be substantially free of sulfur, nitrogen, metals and other knownpoisons. These poisons can be removed by first using conventionalhydrofining techniques, then using sorbents to remove the remainingsulfur compounds and water.

As mentioned above, the catalyst of the present invention exhibits alonger run life with heavier feedstocks, e.g., containing at least 5 wt% C₉ + hydrocarbons, than similar catalysts having been subjected to adifferent treatment. For example, if a L zeolite catalyst is reducedand/or pretreated by conventional methods, run lengths with feedscontaining at least 5 wt % C₉ + hydrocarbons, and typically from 5-15 wt% C₉ + hydrocarbons, are comparatively short. The catalyst obtained viathe treatment of the present invention, however, makes it quitepractical to process such feedstocks containing the C₉ + hydrocarbons.

The feed can be contacted with the catalyst in either a fixed bedsystem, a moving bed system, a fluidized system, or a batch system.Either a fixed bed system or a moving bed system is preferred. In afixed bed system, the preheated feed is passed into at least one reactorthat contains a fixed bed of the catalyst. The flow of the feed can beeither upward, downward, or radial. The pressure is from about 1atmosphere to about 500 psig, with the preferred pressure being fromabut 50 psig to about 200 psig. The preferred temperature is from about800° F. to about 1025° F. The liquid hourly space velocity (LHSV) isfrom about 0.1 hr⁻¹ to about 10 hrs⁻¹, with a preferred LHSV of fromabout 0.3 hr⁻¹ to about 5 hrs⁻¹. Enough hydrogen is used to insure an H₂/HC ratio of up to about 20:1. The preferred H₂ /HC ratio is from about1:1 to about 6:1. Reforming produces hydrogen. Thus, additional hydrogenis not needed except when the catalyst is reduced and when the feed isfirst introduced. Once reforming is underway, part of the hydrogen thatis produced is recycled over the catalyst.

The catalyst is a large-pore zeolite charged with at least one GroupVIII metal. The preferred Group VIII metal is platinum, which is moreselective for dehydrocyclization and which is more stable underreforming reaction conditions than other Group VIII metals. The catalystshould contain between 0.1% and 5% platinum of the weight of thecatalyst, preferably from 0.1% to 1.5%.

The term "large-pore zeolite" is defined as a zeolite having aneffective pore diameter of from 6 to 15 Angstroms. The preferred porediameter is from 7 to 9 Angstroms. Type L zeolite, zeolite X, andzeolite Y, zeolite beta and synthetic zeolites with the mazzitestructure are thought to be the best large-pore zeolites for thisoperation. Type L zeolite is described in U.S. Pat. No. 3,216,789.Zeolite X is described in U.S. Pat. No. 2,882,244. Zeolite beta isdescribed in U.S. Pat. No. 3,308,069. ZSM-4, described in U.S. Pat. No.4,021,447, is an example of a zeolite with the mazzite structure.Zeolite Y is described in U.S. Pat. No. 3,130,007. U.S. Pat. Nos.3,216,789; 2,882,244; 3,130,007; 3,308,069; and 4,021,447 are herebyincorporated by reference to show zeolites useful in the presentinvention. The preferred zeolite is type L zeolite.

Type L zeolites are synthesized largely in the potassium form. Thesepotassium cations are exchangeable, so that other type L zeolites can beobtained by ion exchanging the type L zeolite in appropriate solutions.It is difficult to exchange all of the original cations, since some ofthese cations are in sites which are difficult to reach. The potassiummay be ion exchanged with an alkali or alkaline earth metal, such assodium, potassium, cesium, rubidium, barium, strontium, or calcium.Preferably, the total amount of alkali or alkaline earth metal ionsshould be enough to satisfy the cation exchange sites of the zeolite orbe slightly in excess.

An inorganic oxide can be used as a carrier to bind the large-porezeolite. This carrier can be natural, synthetically produced, or acombination of the two. Preferred loadings of inorganic oxide are from5% to 50% of the weight of the catalyst. Useful carriers include silica,alumina, aluminosilicates, and clays.

FIG. 1 is a plot of hydrogen uptake onto catalyst as a function ofpretreatment temperature. As can be seen from this Figure, as thepretreatment temperature is increased, the fraction of hydrogen bound tocatalyst tends to decrease. If the hydrogen uptake onto catalyst isreflective of the fraction of exposed Pt atoms, then one would typicallyexpect a decrease in activity with an increase in temperature. Theextent to which pretreating a large-pore zeolitic reforming catalyst ina reducing environment at various temperatures affects the activity ofthe catalyst will be demonstrated in Examples 1-8. The extent to whichpretreating a large-pore zeolitic reforming catalyst in a reducingenvironment at various temperatures affects the fouling rate of thecatalyst will be demonstrated in Examples 9, 10, 11 and 12.

EXAMPLES EXAMPLE 1

A catalyst, consisting of 0.65% Pt on barium exchanged K-L zeolite, waspretreated by heating the catalyst in hydrogen (P=50 psig, GHSV=9000)from ambient temperature to 900° F. at a ramp of 10° F./hr and held at900° F. for 24 hours. The temperature was adjusted to the desiredreaction temperature and n-hexane was introduced. The hydrogen tohydrocarbon ratio was 5:1. After steady state was achieved, thetemperature was raised to the new desired reaction temperature. Thebenzene production is summarized in the first line in Table 1. At 900°F., the catalyst activity declined from 80% to 75%.

                  TABLE 1                                                         ______________________________________                                        Reduction/                                                                    Treatment  Benzene Yield, wt. %                                               Temp       800° F.                                                                        830° F.                                                                          860° F.                                                                      900° F.                             ______________________________________                                         900° F.                                                                          22%     37%       55%   80-75%                                     1050° F.                                                                          22%     --        59%   82%                                        1100° F.                                                                          34%     54%       66%   87%                                        1150° F.                                                                          28%     --        68%   87%                                        1200° F.                                                                          30%     49%       70%   90%                                        1250° F.                                                                          24%     --        57%   81%                                        1300° F.                                                                           7%     --        30%   59%                                        1350° F.                                                                          12%     27%       45%   73%                                        ______________________________________                                    

EXAMPLE 2

In this case, the same catalyst as used in Example 1 was pretreated byheating the catalyst in hydrogen (P=50 psig, GHSV=9000) from ambienttemperature to 1050° F. at a ramp of 10° F./hr and held at 1050° F. for3 hours. The temperature was then adjusted to the desired reactiontemperature and n-hexane was introduced to achieve a hydrogen to hexaneratio of 5: 1. The benzene production is summarized in line 2 in Table1.

At 860° F. and 900° F. reaction temperatures, the catalyst treated at1050° F. was more active, producing more benzene, than the catalystreduced at 900° F. In addition, the catalyst treated at 1050° F. did notexhibit deactivation at 900° F. Thus, pretreating at a high temperatureof 1050° F. increased the activity and lowered the fouling rate of thecatalyst.

EXAMPLE 3

In this case, the same catalyst as used in Example 1 was pretreated byheating the catalyst in hydrogen (P=50 psig, GHSV=9000) from ambienttemperature to 1100° F. at a ramp of 10° F./hr and held at 1100° F. for3 hours. The temperature was then adjusted to the desired reactiontemperature and n-hexane was introduced to achieve a hydrogen to hexaneratio of 5:1. The benzene production is summarized in line 3 in Table 1.

At all reaction temperatures, the catalyst treated at 1100° F. was moreactive, producing more benzene, than the catalyst treated at 900° F. Inaddition, the catalyst reduced at 1100° F. did not exhibit deactivationat 900° F. Thus, pretreating at a high temperature of 1100° F. increasedthe activity and lowered the fouling rate of the catalyst.

EXAMPLE 4

In this case, the same catalyst as used in Example 1 was pretreated byheating the catalyst in hydrogen (P=50 psig, GHSV=9000) from ambienttemperature to 1150° F. at a ramp of 10° F./hr and held at 1150° F. for3 hours. The temperature was then adjusted to the desired reactiontemperature and n-hexane was introduced to achieve a hydrogen to hexaneratio of 5:1. The benzene production is summarized in line 4 in Table 1.

At the reaction temperatures of 800° F., 860° F. and 900° F., thecatalyst treated at 1150° F. was more active, producing more benzene,than the catalyst treated at 900° F. In addition, the catalyst treatedat 1150° F. did not exhibit deactivation at 900° F. Thus, pretreating ata high temperature of 1150° F. increased the activity and lowered thefouling rate of the catalyst.

EXAMPLE 5

In this case, the same catalyst as used in Example 1 was pretreated byheating the catalyst in hydrogen (P=50 psig, GHSV=9000) from ambienttemperature to 1200° F. at a ramp of 10° F./hr and held at 1200° F. for3 hours. The temperature was then adjusted to the desired reactiontemperature and n-hexane was introduced to achieve a hydrogen to hexaneratio of 5:1. The benzene production is summarized in line 5 in Table 1.

At all reaction temperatures, the catalyst treated at 1200° F. was moreactive, producing more benzene, than the catalyst reduced at 900° F. Inaddition, the catalyst treated at 1200° F. did not exhibit deactivationat 900° F. Thus, pretreating at a high temperature of 1200° F. increasedthe activity and lowered the fouling rate of the catalyst.

EXAMPLE 6

In this case, the same catalyst as used in Example 1 was pretreated byheating the catalyst in hydrogen (P=50 psig, GHSV=9000) from ambienttemperature to 1250° F. at a ramp of 10° F./hr and held at 1250° F. for3 hours. The temperature was then adjusted to the desired reactiontemperature and n-hexane was introduced to achieve a hydrogen to hexaneratio of 5:1. The benzene production is summarized in line 6 in Table 1.

At the reaction temperatures of 800° F., 860° F. and 900° F., thecatalyst treated at 1250° F. was more active, producing more benzene,than the catalyst reduced at 900° F. In addition, the catalyst treatedat 1250° F. did not exhibit deactivation at 900° F. Thus, pretreating ata high temperature of 1250° F. increased the activity and lowered thefouling rate of the catalyst.

EXAMPLE 7

The catalyst used in Example 1 was pretreated by heating the catalyst inhydrogen (P=50 psig, GHSV-9000) from ambient temperature to 1300° F. ata ramp of 10° F./hr and held at 1300° F. for 3 hours. The temperaturewas then adjusted to the desired reaction temperature and n-hexane wasintroduced to achieve a hydrogen to hexane ratio of 5:1. The benzeneproduction is summarized in line 7 in Table 1.

At all reaction temperatures, the catalyst treated at 1300° F. was lessactive, producing less benzene, than the catalyst reduced at 900° F.

EXAMPLE 8

The catalyst used in Example 1 was pretreated by heating the catalyst inhydrogen (P=50 psig, GHSV=9000) from ambient temperature to 1350° F. ata ramp of 10° F./hr and held at 1350° F. for 8 hours. The temperaturewas then adjusted to the desired reaction temperature and n-hexane wasintroduced to achieve a hydrogen to hexane ratio of 5:1. The benzeneproduction is summarized in line 8 in Table 1.

At all reaction temperatures, the catalyst treated at 1350° F. was lessactive, producing less benzene, than the catalyst reduced at 900° F.

EXAMPLE 9

A comparison of catalyst activity and fouling rate after reduction at500°-900° F. and 500°-1050° F. was made as follows.

In the first case, eighty cubic centimeters of catalyst consisting of0.65 wt. % platinum on barium exchanged, L zeolite, 1/16 inch extrudateswere charged to a one-inch diameter reactor. The catalyst was dried byheating to 500° F. in dry nitrogen flowing at a rate of 12 cubic feetper hour. Catalyst reduction was then initiated at 500° F. by replacingthe nitrogen with dry hydrogen (preferably containing <ppm water)flowing at the same rate. After an hour at 500° F., the temperature wasraised in stepwise fashion to 900° F. and maintained at 900° F. for 12hours to complete the catalyst reduction and dryout. The catalyst wasthen cooled to 800° F. for feed introduction.

In the second case, the same procedure was used except that after theinitial reduction at 500° F. for one hour, the temperature was raised10° F./h to 1050° F. The catalyst was then maintained for two days at1050° F. in flowing hydrogen before cooling to reaction temperature. Thegas flow rate was 12 ft³ /hr throughout.

The feed for the catalyst performance test was a hydrotreated raffinatefrom an aromatics extraction unit consisting of 8.5% C₅, 59.5% C₆, 26.3%C₇, and 5.8% C₈ ⁺ compounds on a weight basis. This feed was alsocharacterized as 85.8% paraffins, 6.8% naphthenes, 6.7% aromatics, and0.7% unknowns by weight. The test was carried out at a feed rate of 1.6liquid hourly space velocity, 100 psig, and a hydrogen to feed molarratio of 3.0. The catalyst bed temperature was adjusted as the runprogressed to maintain 42 wt % aromatics in the C₅ ⁺ product. Thecombined hydrogen and naphtha feedstream was treated to reduce itssulfur content to less than 5 ppb.

The results of the test runs are shown in FIG. 2. The catalyst foulingrates were calculated by a least squares fit of the data obtained after200 hours onstream. The catalyst reduced/treated at 500°-1050° F. hadabout one-fourth the fouling rate of the catalyst reduced at 500°-900°F. (0.005 versus 0.020° F./h). The-start-of-run temperatures obtained byextrapolating the least squares line back to start-of-run were 852° F.and 847° F., respectively. The yield of C₅ ⁺ product was 85 LV % of feedin both cases. Assuming the fouling rate is constant and the end-of-runaverage catalyst temperature is 935° F., the projected run length isabout two years for the catalyst treated at 1050° F. compared to aboutsix months for the catalyst treated at 900° F.

EXAMPLE 10

This example shows that the decreased coking tendencies of the hightemperature reduced catalyst make it possible to carry out a reformingprocess under previously impractical conditions. Compared to Example 9,the liquid hourly space velocity was increased to 1.7, thehydrogen/hydrocarbon ratio was reduced to 2.0, the pressure wasincreased to 130 psig, the aromatics content of the C₅ ⁺ product wasincreased to 72 wt. %, and a heavier feed was employed. Each of thesechanges would be expected to increase fouling.

A feed containing 2.7% C₅ and lighter, 8.5% C₆, 49.4% C₇, 30.8% C₈, and8.7% C₉ ⁺ components was reformed over the 500°-1050° F. reducedcatalyst from Example 9. The feed was further characterized ascontaining 66.6% paraffins, 22.6% naphthenes, 10.5% aromatics, and 0.25%unknowns. Over a period of about 400 hours, the fouling rate under theseconditions was 0.018° F./h which corresponds to more than six months runlength.

EXAMPLE 11

In order to limit catalyst deactivation during the high temperaturetreatment, it is important to control water vapor concentrations. Thisis especially important in a commercial unit wheregas-hourly-space-velocities are limited by compressor size. It ispossible to limit the exposure of the catalyst to water vapor at hightemperatures by using dry hydrogen, measuring the moisture levels in thereactor effluent, setting target values for each temperature range, andlimiting the rate of heatup to stay within the target moisture levelranges. A commercial high temperature treatment was simulated in a smallpilot plant as follows.

Eighty cubic centimeters of 1/16-inch catalyst extrudates were chargedto a one-inch diameter tubular reactor. The catalyst comprised 0.65 wt %platinum, barium exchanged L-zeolite, and a binder. The reactor washeated by a three-zone electric furnace. Catalyst bed temperatures weremeasured by six thermocouples located in an axial thermowell. Thereaction system comprised: the reactor, a chilled liquid-gas separator,a moisture analyzer probe, a compressor, a recycle-gas drier, and arecycle gas flowmeter. The moisture analyzer measured the moisturecontent in the recycle gas before or after the drier. The drier wascharged with 4 Å molecular sieves.

The unit was pressurized to 70 psig with dry nitrogen containing lessthan 10 ppm water. The compressor was started. Nitrogen addition wascontinued in order to produce an off-gas stream and purge the system ofoxygen. After two hours, the nitrogen addition rate was reduced untilthere was only a small off-gas stream. The gas circulation rate wasadjusted to maintain a gas flow over the catalyst bed corresponding to aGHSV of about 1000 h⁻¹. The catalyst was further dried by heating thereactor to 500° F. Water in the reactor effluent was removed by a drier,so that the recycle gas contained less than 10 ppm water. Thetemperature was held at 500° F. until the moisture content of thereactor effluent gas dropped below 100 ppm.

The make-up gas was then switched from nitrogen to dry hydrogen and theunit was pressurized to 100 psig. After reaching 100 psig, the hydrogenaddition rate was adjusted to maintain a small gas bleed. The gascirculation rate was adjusted to obtain a GHSV of about 1000 h⁻¹.Following hydrogen addition, there was an increase in the water contentof the reactor effluent due to catalyst reduction. This water wasremoved from the recycle hydrogen stream by the recycle-gas driers. Thereactor-inlet gas contained less than 10 ppm water. The reactortemperature was held at 500° F. until the water in the reactor effluentagain dropped below 100 ppm. The reactor temperature was then raised 10°F./h to 900° F. Temperature was held at 900° F. until the moisture levelin the reactor effluent dropped to 20 ppm. The reactor was then heatedto 1100° F. at a rate of 10° F./h. After a 3-hour hold at 1100° F., thetemperature was dropped to 800° F. and the naphtha feed was introduced.

The high temperature treated catalyst was tested with several feeds atseveral different conditions. When tested at the conditions used inExample 9, but with a heavier feed, the fouling rate was 0.007° F./hcompared to 0.025° F./h for the same catalyst reduced in the temperaturerange of from 500° to 900° F.

EXAMPLE 12

A potassium L-zeolite catalyst also surprisingly benefits from a hightemperature hydrogen treatment. Platinum was loaded onto a bound, 20-40mesh, K-L zeolite support using the incipient wetness impregnationmethod and an aqueous Pt(NH₃)Cl₂ --H₂ O solution. The impregnatedmaterial was oven-dried at 120° F. overnight and calcined at 500° F. forfour hours.

In three separate experiments, one-gram of the calcined material wasloaded into a 3/16" I.D. tubular microreactor. In each case, thecatalyst was dried by heating to 500° F. in nitrogen flowing at a rateof 550 cc/min. In the first experiment, the catalyst was reduced in 550cc/min of hydrogen while the reactor temperature was heated from 500° to900° F. at a rate of 10° F./h. In the second and third experiments, theactivation procedure was the same except that the final temperatureswere 1100° and 1150° F. respectively The catalyst samples were held attheir peak temperature for three hours, then cooled to 875° F. fortesting.

A C₅ -C₈ raffinate stream from an aromatics extraction unit was reactedin the presence of hydrogen over each catalyst sample. Reactor effluentanalyses were obtained by gas chromatography. Conversion and selectivitywere calculated from the feed and product analyses. Table 2 shows thatthe stability of the Pt-K-L zeolite catalyst was significantly improvedby high temperature reduction. Conversion after about six days on-streamwas significantly higher for the catalysts treated at 1100° or 1150° F.than when the reduction temperature was limited to 900° F. "Conversion"refers to the conversion of C₆ ⁺ feed components and "selectivity" isthe selectivity for aromatics and hydrogen production. Both arecalculated on a weight basis.

                  TABLE 2                                                         ______________________________________                                        Catalyst Reduction                                                                         Hours on  Conversion Selectivity                                 Temperature  Stream    Wt %       Wt %                                        ______________________________________                                        500-900° F.                                                                          3        62.3       87.3                                                     145       36.1       89.0                                        500-1100° F.                                                                         6        61.8       88.5                                                     146       50.5       90.9                                        500-1150° F.                                                                         5        53.7       89.0                                                     147       44.8       90.6                                        ______________________________________                                         Run Conditions:                                                               WHSV = 4.4, H.sub.2 /HC = 5.0, Temp. = 875° F., Pres. = 50 psig   

What is claimed is:
 1. A method of pretreating a reforming catalystcomprising a large-pore zeolite containing at least one Group VIIImetal, wherein said catalyst is treated with hydrogen gas in thetemperature range of from 1025° to 1275° F. while maintaining the waterlevel of the effluent gas below 200 ppm.
 2. The method according toclaim 1, wherein the moisture level of the effluent gas is maintainedbelow 100 ppm in the temperature range of 1025°-1275° F.
 3. The methodaccording to claim 1, wherein the temperature is increased at a ratebetween 5° and 50° F. per hour.
 4. The method according to claim 1,wherein the temperature is increased at a rate between 10° and 25° F.per hour.
 5. The method according to claim 1, wherein the temperature isslowly increased in a stepwise fashion.
 6. The method according to claim1, wherein the temperature is slowly increased in a ramping fashion. 7.The method according to claim 1, wherein, before a temperature of 1025°F. is reached, said catalyst is treated with hydrogen gas while slowlyincreasing the temperature from 900° to 1025° F., and wherein the waterlevel of the effluent gas is maintained below 200 ppm.
 8. The methodaccording to claim 1, wherein, before a temperature of 1025° F. isreached, said catalyst is treated with hydrogen gas while slowlyincreasing the temperature from 900° to 1025° F., and wherein the waterlevel of the effluent gas is maintained below 100 ppm.
 9. The method ofpretreating a reforming catalyst according to claim 1, wherein thecatalyst comprises platinum Group VIII metal is platinum.
 10. The methodaccording to claim 9, wherein the amount of platinum is in the range offrom 0.1 to 1.5 wt %.
 11. The method of pretreating a reforming catalystaccording to claim 1, wherein said catalyst comprises a large-porezeolite is selected from the group consisting of zeolite X, zeolite Y,and type L zeolite, beta zeolite, or zeolites having the mazzitestructure.
 12. The method of pretreating a reforming catalyst accordingto claim 1, wherein said catalyst comprises a type L zeolite containingplatinum.
 13. The method of pretreating a reforming catalyst accordingto claim 1 wherein said catalyst comprises a large-pore zeolite and aninorganic binder.
 14. The method of pretreating a reforming catalystaccording to claim 13, wherein said inorganic binder is selected fromthe group consisting of silica, alumina, aluminosilicates, and clays.15. The method of pretreating a reforming catalyst according to claim 1,wherein said catalyst comprises a large-pore zeolite containing at leastone Group VIII metal and an alkali or alkaline earth metal selected fromthe group consisting of potassium, barium, strontium, calcium, sodium,rubidium, and cesium.
 16. The method of pretreating a reforming catalystaccording to claim 15, wherein said alkaline earth metal is barium andwherein said Group VIII metal is platinum.
 17. The method of pretreatinga reforming catalyst according to claim 15, wherein said catalyst hasfrom 0.1% to 35% by weight of alkali or alkaline earth metal and from0.1% to 5% by weight platinum.
 18. The method of preheating a reformingcatalyst according to claim 16, wherein said catalyst has from 0.1% to35% by weight barium and from 0.1% to 5% by weight platinum.
 19. Themethod of pretreating a reforming catalyst according to claim 1, whereinsaid catalyst comprises:(a) a type L zeolite containing from 0.1% to 5%by weight platinum; and (b) an inorganic binder selected from the groupconsisting of silica, alumina, aluminosilicates, and clays.
 20. Themethod of pretreating a reforming catalyst according to claim 1, whereinsaid catalyst comprises:(a) a type L zeolite containing from 0.1% to 35%by weight alkali or alkaline earth metal and from 0.1% to 5% by weightplatinum; and (b) an inorganic binder selected from the group consistingof silica, alumina, aluminosilicates, and clays.
 21. The method ofpretreating a reforming catalyst according to claim 1, wherein saidcatalyst comprises:(a) a type L zeolite containing from 0.1% to 35% byweight barium and from 0.1% to 5% by weight platinum; and (b) aninorganic binder selected from the group consisting of silica, alumina,aluminosilicates, and clays.
 22. The method of pretreating a reformingcatalyst according to claim 1, wherein said catalyst comprises:(a) atype L zeolite containing from 0.1% to 35% by weight barium and from0.1% to 5% by weight platinum; and (b) an inorganic binder selected fromthe group consisting of silica, alumina, aluminosilicates, and clays.23. The method according to claim 1, wherein the catalyst issubsequently used to reform a hydrocarbon feedstock containing fromabout 5 to 15 wt % of C₉ + hydrocarbon.
 24. A method of pretreating areforming catalyst comprising a large-pore zeolite, wherein saidcatalyst is treated with hydrogen while slowly increasing thetemperature to a temperature between 1025° and 1275° F. such that themoisture level in the reactor effluent is maintained at a low enoughlevel to permit the number of acid sites on the catalyst to beadequately reduced so as to achieve a catalyst exhibiting longer runlengths.
 25. A reforming process comprised of contacting a hydrocarbonfeed with a large pore zeolite catalyst containing at least one GroupVIII metal, wherein the catalyst has been treated by the methodaccording to claim 1.